1. Field of the Invention
The present invention relates to non-destructive inspection of composite structures.
2. Description of the Related Art
A critical need in the aircraft industry is the non-destructive inspection (NDI) of composite structures assembled with adhesive bonds. The typical means of testing bonded structures is a destructive proof test that may validate an assembly process, but does not prove that any given assembly is sufficiently strong to have a high probability of not failing in service.
In particular, it is of interest to find weakly bonded layers in multi-layer carbon-fiber/resin-matrix skins and internal members. These weak bond areas are not observable with conventional ultrasound or thermal techniques because the bond between layers, while weak, is intact or in contact and no delamination area exists that would be visible to standard inspection techniques. These defective bonds could arise from (1) improper preparation of the surfaces to be bonded; (2) improper mixing, application, and/or curing of the adhesive; or (3) contamination of the surfaces before bonding. In the past, electron beam pulses have been used to deposit energy in depth in a composite structure on a time scale that is short compared to the acoustic transit time through the thickness of the material. This energy deposition results in a release wave propagating into the material from both surfaces. When the release waves meet they produce a tensile stress which can be used to test the strength of the bond at that point in the material. Unfortunately, the electron beam generator is massive, therefore not portable, and the output beam cannot be conveniently moved around the surface of an aircraft to test the skin or other structural members.
Patent Application No. US 2002/0129659 A1 and US 2003/0079552 A1 provide examples of known methods and apparatus.